Genetics 1 Flashcards
Gene
a heritable factor that consists of a length of DNA and controls the synthesis of one polypeptide chain (a segment of the DNA)
Gene locus
the specific and fixed position of a gene along a chromosome/DNA
Homologous chromosomes vs sister chromatids
have the same bending pattern but can have different gene alleles
have identical genes, identical DNA copies attached at the centromere
Allele, homozygous and heterozygous chromosome
a variation of a single gene that differs from other alleles of the same gene by a few base pairs
dominant and recessive alleles, heterozygous are those with different alleles, homozygous chromosome with only one
Single nucleotide polymorphism (SNPs)
alternative allele, differing in just a few base pairs from the original gene and this difference doesn’t affect the structure – the result of base-substitution mutations
Highly repetitive sequence (HRS)
at telomeres and the centromere (gets lost in the DNA replication)
Genome
the complete DNA of a cell (includes both coding and non-coding regions), genome size expressed as the number of base pairs (in humans 3*10^9, number of genes = 23000)
Ploidy
the number of complete sets of chromosomes and their genetic information – a cell can be haploid (n) like gametes or diploid (2n) like somatic cells where n is the number of chromosomes in one set
Karyotype
describes the number and appearance of the chromosomes in an eukaryotic cell – members of the same species usually have the same karyotype if there is no mutation
Karyogram, karyotyping
micrograph of metaphase chromosomes where they are arranged into homologous pairs beginning with autosomes and ending with gonosomes – used for gender prediction and detection of chromosomal abnormalities
the procedure of obtaining the karyogram
Autosomes and gonosomes in humans
44 (22 pairs) autosomes (body chromosomes) and two (XX or XY) gonosomes (sex chromosomes)
Meiosis
division of diploid somatic cells in sex glands to produce haploid gametes using the mechanism of reducing the number of chromosomes to half the number found in somatic cells and promote genetic variation
Miosis steps
Prophase I (the longest phase of meiosis (90% of time required, lasts for hours or days))
Metaphase I (bivalents line up at the equator – random orientation of each pair increases genetic variability)
Anaphase I (bivalents split, each homologous chromosome goes to opposite pole)
Telophase (two clusters formed, nuclear membrane reforms, chromosomes partly uncoil)
Cytokinesis I (possible brief interphase with no DNA replication)
Prophase II Metaphase II Anaphase II (sister chromatids separated) Telophase II Cytokinesis II – produced four genetically different haploid gametes
How does random bivalent orientation increase genetic variability
since each bivalent can orient itself in 2 ways, there are 2^n genetically different gametes that can be produced in each organism where n is the number of bivalents (2^23 in humans)
Synopsis
pairing up of not completely condensed homologous chromosomes to exchange genes between their non-sister chromatids (crossing over or recombination) – tetrads (4 chromatids)/bivalents (2 chromosomes) formed
Chiasma
the place where non-sister chromatids cross over
How does crossing over increase genetic variability
results in the recombination of linked genes or the production of new combinations of linked genes in gametes (otherwise only two different gametes would be produced (n=2))
Genetic variation can be increased by:
I| Crossing over
II| Random orientation of bivalents
III| Random fertilization of gametes
State, in each phase of both mitosis and meiosis, how many and what type (single or double) of chromosomes are present in a human cell
… (table)
Mitosis vs meiosis
mitosis is a means of asexual reproduction and meisosis is used to produce gametes which are used in sexual reproduction – only meiosis has crossing over/synopsis and bivalents (homologous pair recognize each other) – mitosis produces two identical daughter cells (somatic) and meiosis four genetically different gametes – in mitosis homologous pairs are blind to each other while they recognize and pair up in meiosis
Linked genes
genes most likely to be inherited together (usually not separated in crossing over because of their closeness – otherwise DNA would entangle and mutation would occur)
Nondisjunction
when bivalent (homologous chromosomes)/sister chromatids don’t separate during anaphase I/II and it leads to aneuploidy (mutated gametes)
Risk factors for nondisjunction
when a parent (especially the mother) is older – the longer the homologous chromosomes are joined (frozen in bivalents in eggs), the more difficult it is to separate them – if an older egg gets fertilized, the probability of it being mutated is greater
Chromosomal mutations, and types
changes in structure/number of chromosomes – chromosomal aberration, aneuploidy, and polyploidy – a human can only survive a chromosomal mutation on the 13th, 18th, 21st, and sex chromosome pairs
Chromosomal aberration
e.g. cri-du-chat syndrome – one arm on the fifth chromosome missing
Aneuploidy
when one pair of a chromosome has a chromosome surplus or missing so the number of chromosomes in a gamete is n+1 or n-1 instead of n, e.g. trisomy (three chromosomes in one pair – most common in gonosomes) like the Down syndrome (trisomy 21) and Klinefelter syndrome (XXY syndrome)
Polyploidy
when each homologous pair has an extra chromosome so a gamete has 3n, 4n…number of chromosomes, e.g. triploidy (all pairs have three chromosomes)
DNA mutations, and types
permanent structural changes to the DNA of a gene that can be inherited – addition, deletion, substitution (doesn’t have to be detrimental because of degeneration property of the genetic code), translocation, and inversion
Types of mutations by how they affect the amino acid produced from the gene
same-sense mutations don’t change the amino acid, non-sense mutations code for a stop codon instead of an amino acid, and mid-sense mutations result in a codon that codes for a different amino acid
Sickle-cell anemia
an example of substitution mutation from GTG to GAG which results in glutamic acid being coded for instead of valine which results in a mutated Hb (hemoglobin) with a different secondary and tertiary structure and therefore function – it can carry less oxygen than normal Hb so erythrocytes cannot carry their function properly and they are deformed in such a way that they stick together and form clumps (impairs the blood flow and can even lead to a stroke)
Connection between Hb genotype and malaria
Hb^n Hb^n genotype is healthy but, if infected, suffers badly from malaria, Hb^s Hb^s cannot be infected by malaria as plasmodium can only survive in healthy erythrocytes but it suffers badly from anemia, and the Hb^s Hb^n genotype suffers mild anemia and is protected from malaria (plasmodium cannot survive in their blood due to some chemical imbalances
What number of nucleotides is best to be inserted or deleted as to cause the least consequences to the nucleic strand?
three or a multiple of three – only one a-a gets added or removed – if any other number gets inserted/deleted the entire reading frame gets changed that is all a-a are different (frameshift mutation)
Examples of positive mutations
development of lighter skin (less melanin needed to protect the skin from light) in places with less sun enables greater absorption of UV light which turns previtamin D into vitamin D – lactase production in adults (should be limited only to babies)
Gene knockout, and an example
the latest technique to study the function of unknown genes – organisms with dysfunctional variations of a gene of interest are made and the change in the phenotype reveals the gene’s function – e.g. p53 (higher tumor incidence and age faster)
CRISPR (Cluster Regularly Interspaced Palindromic Repeats)
new gene editing technology based on the prokaryotic immune system that helps them evade viral infection – they remember (molecular memory) an old phage infection by taking a part of its DNA (spacer) each time – different spacers are separated by repeats and once a virus infects the organism a specific spacer and its repeat get transcribed to form gRNA (guide RNA) which cuts the viral DNA in order to deactivate it – CRISPR enables us to insert/delete sections of the DNA into cells with extreme precision
Particulate inheritance
traits are determined by genes that separate during meiosis and reunite upon fertilization – they don’t lose their integrity but may be overshadowed by the intensity of another gene (dominant and recessive genes)
The seven alternative phenotypic traits in pea plant
flower color and position, seed color and shape, pod color and shape, and height
Phenotype vs genotype
set of all expressed traits (characteristics) determined by the genotype – alleles possessed by an organism (genetic potential regarding a certain trait)
Polygenic vs monogenetic trait
alternative vs multiple alleles
homozygous vs heterozygous
trait determined by multiple genes – trait determined by a singular gene
only two alleles (dominant or recessive) or more than two (e.g. erythrocyte blood group)
has only one allele (no hidden) – has two different alleles
Filial generation, what are we using to examine it?
the first generation whose genotype and phenotype we’re following – punnett grid is used to look at their offspring’s genotypic and phenotypic ratios
